First of all, salt is one of the substances you can really "taste". Your tongue can only taste salt, sweet, bitter and sour. Everything else is "flavor" and mediated by the sense of smell. Traumatic head injury is a common cause of the loss of sense of smell. A person who looses the sense of smell will complain about not being able to "taste" food. It's the combination of tasting the salt with the tongue's taste buds while also processing the flavor. Besides, sodium is an essential electrolyte. The body doesn't "waste" sodium like it does potassium, so there's no need, under normal circumstances, to overdo it.

Explains why as a peanut butter fanatic, i hate salt free peanut butter.